Actually Andy, suppose you owned the book, and a digital scanner. Then you could scan the book into a pdf file and share it with whomever you pleased, say 100 friends, as easy sending an email. Now assume that doing so is sinful. If it is then can’t be just an accidental property of your action that makes it sinful but must be something intrinsic to the act of sharing the material that makes it so. But then you could, if you chose, take the book from friend to friend, loaning it to each of them one at a time until all 100 had gotten their use of it. We would not think it illegal or sinful for you to do so. But then how are the cases dissimilar? It can only be because your friends are keeping a copy in the pdf case. So then, imagine that your friends, to whom you loan the book also have photocopiers. Is it a sin for them to photocopy the book you loaned them? If you say yes, then you must tells us why it is wrong for them to mechanically photocopy the book, but not for them to copy the book with their brain (which is in essence what it is to read a book, i.e., to make a mental copy of the thing read). How is the choice of copying mediums morally salient (brain tissue in the temporal lobes vs. paper)? You are copying my words right now, just by reading this post, and you may repeat what you read to others, and that could be a sin (which seems to absurd, right?).
I don’t know what to say other than to trust in Holy Spirit and guidance and counsel of one’s confessor on the matter.
I do know this. The Gospel would not have spread as far as it did, as fast as it did, with today’s publishing/copyright Laws. Unless of course, the Church was Savy enough to endorse and utilize public filesharing.